This invention relates to a chroma-key tracking apparatus.
A chroma-key tracking apparatus is used to produce a special effect by compressing a television picture to a small size corresponding to a chroma-key frame, and inserting the compressed television picture into another television picture. An apparatus of this kind is disclosed in U.S. Pat. No. 4,200,890. In the apparatus disclosed in this patent, a frame position signal representing the frame size to which the television picture is compressed is generated based on a chroma-key signal, and a picture compressing circuit compresses a television picture in response to this frame position signal, or to a frame position signal which is a little larger. The compressed television picture is supplied to a mix-keyer circuit, and combined with another television picture in response to the chroma-key signal.
In the disclosed apparatus, minimum and maximum positions in the horizontal and vertical directions with respect to the chroma-key signal are detected for generating the frame position signal in such a manner as shown in FIG. 4. This method can be practiced by a comparatively simple circuit construction. However, when an unwanted partial spot noise is included in the chroma-key key signal, a chroma-key frame which includes this partial noise spot is inevitably generated. When such an erroneous detection occurs, the television picture is not compressed to the size that corresponds to the desired size represented by the chroma-key signal in the picture compressing circuit, so that the desired special effect is not obtained. When the key signal, especially the chroma-key signal, is generated by discriminating a hue difference, the partial spot noise is likely to occur in certain television pictures. Furthermore, the partial spot noise also occurs intermittently, so that the stable chroma-key tracking operation can not be performed.